1. Field of the Invention
Determining the presence of cracks or flaws in materials and solid structures is important in many fields. In order to reliably predict failures, it is often desirable to be able to determine the size and nature of cracks inside the material through Nondestructive Evaluation (NDE) methods. However, it remains a difficult problem to accurately visualize the cracks, which can be advantageous in repairing the structure or for the implementation of future improvements.
2. Discussion of the Prior Art
Ultrasonic methods are well established in the NDE field, where ultrasonic waves are propagated through the material and the reflected surface wave, or so called xe2x80x9cRayleigh wavesxe2x80x9d, are analyzed to determine the presence of flaws. U.S. Pat. No. 5,894,092 to Lindgren, et al, for example, teaches a method for obtaining near-surface characteristics of a material through the generation of a series of single frequency Rayleigh waves, detecting the reflected Rayleigh waves and determining their velocities in order to obtain the depth from the surface of one or more characteristics of the material. U.S. Pat. No. 5,974,889 to Trantow, on the other hand, discloses a method for ultrasonically scanning a surface with a ultrasonic waves from an apparatus comprising a plurality of transducers to detect characteristics on a material""s surface. The inventor of the present invention was one of the named inventors of U.S. Pat. No. 5,859,370, granted on Jan. 12, 1999 (hereinafter xe2x80x9cthe ""370 patentxe2x80x9d). The ""370 patent discloses a method of detecting and sizing small cracks in roots between crests in stud bolt threads. The ""370 patent is hereby incorporated herein in its entirety. It teaches a method comprising the steps of radiating an ultrasonic wave into the flank of the bolt threads, detecting the reflected signal which is one of the regularly spaced signals reflected from the threads, detecting a Rayleigh wave signal from the Rayleigh wave propagating along the crack, measuring the elapsed time from the reflected signal and ending at the Rayleigh wave signal, and calculating the crack size by the elapsed time. Digital signal processing means, including modified wave shaping, dynamic predictive deconvolution and a combination thereof, are utilized to efficiently detect the Rayleigh waves. By this method, a crack as small as 1-2 mm can be detected inside a material.
In conventional ultrasonic imaging methods, the specular reflections are usually used for image reconstruction. However, these techniques are not efficient in most cases due to multiple specular reflections or mode converted acoustic waves, resulting in a distorted or inaccurate image that is not representative of the crack. In conventional methods, the resolution is often poor as the wavelength of the ultrasonic signal approaches the dimensions of the defect. This would occur when the defect sought to be imaged is very small. It is a goal of the present invention to obtain higher resolution and higher accuracy in the reconstructed image than possible with conventional methods through the use of Mode Converted Rayleigh Waves (MCRW).
In the present invention, a ultrasonic signal transmitter and an array of signal receivers are utilized to measure cracks and flaws within a solid material. MCRW is used for ultrasonic image reconstruction and the specular reflection is used for range reference in the image. To effectively detect the MCRW, small aperture transducers are used and the transducers are circularly arranged around the transmitter. In addition, deconvolution methods are used to suppress the effect of specular echo on the MCRW.